Eye color

The eye color is determined by pigments in the intrinsic layer ( stroma) of the iris ( Iris Latin ). It is the result of complex processes in which several genes are involved.

• 7.1 Eye Color in leucism and albinism

Gallery

Human Eye, gray-green with orange red elements

Human Eye, green-brown

Human Eye, gray

Human Eye, Blue

Human Eye, brown

Human Eye, light blue

Degree of pigmentation

The root in people of European ancestry is mostly blue, here the stroma of the iris is pigmented and the color comes largely through the thin pigment layer ( epithelium pigmentosum ) on the back of the iris into existence. This is especially evident in neonates with light skin color, the first usually have blue eyes appearing because with them the pigment melanin, which is responsible for the color of the pigments, is still scarce. Babies from Asia, Africa and Latin America are born dependent on the genes of the parents usually already with brown eyes. Whether a newborn reserves the blue or non- brown eye color, decides in humans depending on Genlage only at the end of the first year of life and may even change during puberty yet. Even domestic cats usually come with bright blue eyes to the world and develop the subsequent eye color after about three months. Different breeds (eg Siamese ) and a few dog breeds ( eg huskies ) have also as adults on blue eyes.

By incorporation of the braunfärbenden melanin in the iris own layer, a characteristic eye color, depending on the amount of pigment over gray, yellow, green to brown, with a correspondingly high amount of melanin even to black, ranging forms. This correlates in humans usually with skin and hair color. So fair-skinned and blond people tend to have more blue eyes, while dark-skinned with dark hair usually have a brown iris coloration. It must be said also that the skin tone is only weakly influenced by the causal alleles ( gene variants ) for eye and hair color, apart from those alleles for blue eyes and red hair, which cause a lightening of skin tone.

Rarely, it may occur that only in one eye pigments store ( iris heterochromia ). Then the person has different eye colors ( mostly green and blue). Also it can be observed that the color of eyes associated with the sunlight. People in the Arctic usually have blue eyes, because there is hardly any direct sunlight. Brown coloration of the iris are typical of many mammals.

Brightening of the eyes by albinism and leucism

Also lack the pigment in the posterior epithelium of the iris and in the choroid, the blood vessels of the eye fundus shine through and the iris looks red. Such Red eyes occur in albinism and leucism occasionally, weaker forms of albinism, leucism and various Scheckungsgene but can brighten up even darker eye colors to blue or light brown. This iris lightening are possible both in birds and in mammals and reptiles.

It is often mistakenly assumed that one could distinguish animals with albinism from those with leucism on the eye color, but by know albino red, Leucistic animals at the blue eye color. One actually finds many examples of red-eyed animals with albinism, blue-eyed leucistic animals with a completely white coat. Although albinism melanin disorder always affects the whole animal and in some melanocytes leucism often find the shortest path to eye, there are still blue-eyed albinos like horses of color Cremello. Similarly, there are Leucistic animals with red or pink eyes like mice with mutations in the Mitf gene.

Occasionally there may be compression of the connective tissue in the stroma to the formation of white spots. Such a wall eye is more common in spotted breeds of dogs ( Dalmatian, German mastiff) and horses. It is not, this is a disease. These white spots may cover the whole iris, so the whole eye appears white (glass eye).

Heredity

On the inheritance of eye color at least three different genes are involved, their exact functions are not yet understood completely. What color eyes will the child depends on the combination of genes that are passed down from both parents to the child.

The majority of neonates with white skin color is blue initially appearing eyes.

The published in 1907 by GC Davenport and CB Davenport 1 gene model explains the inheritance of eye color easiest, but describes the complex reality of inheritance only insufficient, as we know today. According to this model there is only one gene that determines eye color (brown or blue). This gene is diploid, so that is present in two copies. In the Davenport model is available for every copy of two forms ( alleles), one for blue eyes (that is no capacity for melanin production ) and an allele for brown eyes (ie ability of melanin production ). If the father has for example two brown eye color allele, the mother, however, two blue eye color alleles, the child will ever one brown and one blue allele obtained.

In Davenport model, the eye color, the man who later depends, depend on which allele is dominant and which is recessive. Where, brown eyes are dominant, while blue eyes are recessive. That is, in the example above, the child would have brown eyes. In another generation, but both genes can be passed on equal footing. If the above child 's father, whose wife also brown eyes ( with a blue and a brown allele ), so the probability is 25 % that a blue-eyed child arises, the two blue genes has and can not pass brown genes. With 75 - % probability of a brown-eyed child is created. Overall, the probability of that at least one of the two alleles is blue at 75 %.

The described Davenport model is now considered outdated. In reality, the inheritance of eye color is brown / green / blue / gray by more than one gene controlled. This Bey2 ( short for. Brown eye 2) the function of genes and applies gey ( green eye ) as backed up. For the gene Bey2 alleles exist for brown and blue eyes, for the gene gey are alleles for green and blue eyes. The following dominance order are: From top to bottom decreases the dominance that is recessive to.

Each of these genes is present diploid. This recessive genes can be passed on to the child generation as well as the above -mentioned simplified Davenport model, even if these genes do not (ie, the external appearance ) determine the phenotype in the parental generation.

It is from the effect of other genes, which affect the different shades of colors and the expression (ie the reading ) of the other genes involved in control since the genes outlined above Bey2 and gey not explain all cases of inheritance. Such is the case brown-eyed children with parents with blue or green eyes explained beyond the genes described above ( a) by mutations in the male germ line, (b) by the action of other (possibly regulating ) genes and ( c ) the complexity of the melanin production. This process consists of a chain of chemical reactions. To illustrate the scheme is greatly simplified agency before the process of melanin production as a result of the conversion of substance A to B and B to melanin. Now, if (conversion of A to B) genetic does not work with a parent, the first process, but the subsequent reaction B can expire after melanin and that A can be transformed into B by the other parent, but not B converted to melanin because of existing genes can be, then no parent has brown eyes ( because you can run the full chain reaction in either parent ). A child can but the ability of the conversion of A to B and from the other parent the ability of the conversion of B inherit from a parent to melanin, so it has brown eyes. This explains, first, why the eye color of children may differ significantly from the color of both parents not only due to diploid chromosome sets. On the other hand it is also clear that the eye color of children does not have to be necessarily come in the grandparents' generation (or of other generations ) to the expression.

Dissemination

About 90 percent of all people around the world have brown eyes, including the vast majority of people of non-European descent.

The rest is split between blue, green and gray, with green with 2-4% is the rarest eye color. One of the few exceptions some Iranian peoples among whom Grünäugigkeit is widespread. Very few brown-eyed people are in the Baltic Sea region. Estonia is the country in which blue eyes occur most frequently ( 99% of the population).

According to the Genforschers Hans Eiberg of the University of Copenhagen, the gene switch for the development of blue eyes had revealed only six to ten thousand years ago estimated by mutation. This change is so specific that he suspects that all blue-eyed people today are descended from the same people. Geographically, he situates this north-east of the Black Sea.

A survey conducted in 2013 Genome analysis of a good 30 -year-old man ( Braña 1), who had lived in the north of the Iberian Peninsula around 7000 years ago and whose bones were discovered in late 2006 in the La Braña - Arintero Cave in the Cantabrian Cordillera, the earliest genes previously found for blue eyes (the same mutation of the HERC2 gene on both chromosomes as in modern humans ) were. Attention was aroused by the fact that this also genes have been identified for dark skin and hair color, as it is typical for Africans.

Eye color as a biological feature

The eye color and its relationship with skin tone and hair color mainly determined with about the appearance of a person. The eye color is entered as (largely) immutable body feature in the identity card and is usually part of an accurate physical description.